HYPEKBOLIC AND PARABOLIC COMETS. 



fig. 6, an ellipse, which may be considered as representing the 

 form of the orbits of the comets Nos k 15, 6, 9, 12, and 1, of 

 Table VI in the " Handbook of Astronomy." 



If the ellipse represent the orbit of the comet No. 15, the circle 

 a will represent on the same scale the orbit of Neptune. 



If the ellipse represent the orbit of the comet No. 6, the circle b 

 will represent the orbit of Neptune. 



If the ellipse represent the orbit of No. 9, the circle c will 

 represent the orbit of Neptune. 



If the ellipse represent the orbit of No. 12, the circle d will 

 represent the orbit of Neptune. 



If the ellipse represent the orbit of No. 1, the circle e will 

 represent the orbit of Neptune. 



V. HYPERBOLIC COMETS. 



74. In a Table in the "Hand-Book of Astronomy " are given 

 the elements of seven comets which appear by the results of cal- 

 culations made upon the observations to have passed through the 

 system in hyperbolic orbits. 



VI. PARABOLIC COMETS. 



75. Of all the remaining number of comets which have been 

 seen in the heavens and recorded in history, one hundred and 

 sixty-one have been observed with sufficient precision to enable 

 astronomers to determine, with more or less approximation, their 

 parabolic orbits. A table giving the elements of these, with the 

 dates of their appearance, will be found in the eighteenth chapter 

 of the " Handbook of Astronomy." 



VII. PHYSICAL CONSTITUTION OF COMETS. 



76. Comets in general, and more especially those which are 

 visible without a telescope, present the appearance of a roundish 

 mass of illuminated vapour or nebulous matter, to which is often, 

 though not always, attached a train more or less extensive, com- 

 posed of matter having a like appearance. The former is called 

 the HEAD, and the latter the TAIL, of the comet. 



77. The illumination of the head is not generally uniform. 

 Sometimes a bright central spot is seen in the nebulous matter 

 which forms it. This is called the NUCLEUS. . 



The nucleus sometimes appears as a bright stellar point, and 



187 



